In digital optical communication, with increasing transmission capacity in recent years, path relief is required in the event of a failure such as a transmission line break, as well as improved line use efficiency. The bidirectional line switched ring (BLSR) is employed as a means to meet the above requirement. At present, the BLSR is supported in the higher-order paths of an STS (synchronous transport signal) level (>51.8 Mbps) mainly for use in the backbone network. For the purpose of implementation in a subscriber system, it is necessary to provide the BLSR supporting the lower-order VT (virtual tributary) paths (>1.5 Mbps) in future.
In the conventional STS-level BLSR network, in order to avoid misconnection at the time of the path relief, a path alarm signal (AIS: alarm indication signal) is inserted on a misconnected path. This is referred to as squelch operation.
To conduct the squelch operation, a squelch table is introduced in the conventional BLSR network. The squelch table, as shown in FIG. 1, is conveyed in an overhead byte to transfer to each node in the network. With this information, each node can identify a source node and a destination node of each path (hereafter referred to as channel Ch).
In FIG. 1, S denotes the source node ID, while D denotes the destination node ID. Further, {circle around (1)} E(east) W(west) and {circle around (2)} W(west) E(east) denote transfer directions of the squelch table.
In order to prevent the squelch table from failing to structure when a failure occurs in one direction, it is necessary to transfer the identical squelch table in both directions. The squelch table transferred in the direction {circle around (1)} is used when no failure occurs or when a failure occurs in the direction {circle around (2)}, while the squelch table transferred in the direction {circle around (2)} is used when the failure occurs in the direction {circle around (1)}.
Now, hereafter describes an example of structuring the squelch table under an exemplary channel setting condition shown in FIG. 2. The BLSR network shown in FIG. 2 is structured of three nodes having node IDs 1, 2 and 3, respectively, connected by transmission lines in the above-mentioned directions {circle around (1)} and {circle around (2)}.
In the node having node ID=2, a signal on a channel (Ch) 1 being input from the E (east) side is added at the node having node ID=1, and dropped at the node having node ID=2. In this case, the source node ID becomes ‘1’, and the destination node ID becomes ‘2’.
Further, in the squelch table shown in FIG. 1, ‘1’ is set into S1, and ‘2’ is set into D1. Because the signal output from the E side is added at the node having node ID=2, and dropped at the node of interest, ‘2’ is set into S2, and ‘1’ is set into D2.
Similarly, because a signal output from the W side is added at the node having node ID=2, and dropped at the node having node ID=3, ‘2’ is set into S3, and ‘3’ is set into D3. Also, because a signal input from the W side is added at the node having node ID=3, and dropped at the node having node ID=2, ‘3’ is set into S4, and ‘2’ is set into D4. As a result, the squelch table for channel (Ch) 1 in the node having node ID=2 becomes as shown in FIG. 3.
As having been described above, in the conventional BLSR network, it becomes necessary to provide the table shown in FIG. 1 on a channel-by-channel basis.
When a failure such as a line break occurs in the network, each node judges whether each channel can reach the destination node based on the squelch table. If it is determined unable to reach, the node concerned squelches the channel concerned.
In this case, in order to squelch the VT-level channel, if this squelch is performed using an STS squelch table on an STS-level bases after bundling the VT-level channels to the STS, unnecessary service interruption to the network users may be produced.
The reason will be explained as follows, taking an example of the following case: The BLSR network structured of four nodes connected as shown in FIG. 4A. Assume that the communication between the nodes having node IDs 2 and 3 becomes disabled, caused by a failure. In addition, the communication between the nodes having node IDs 3 and 4 (hereafter, the nodes will simply be represented as nodes ID3, ID4, etc.) becomes disabled by a different failure.
In the normal condition shown in FIG. 4A, VT channels A and B are added at node ID4, mapped to a channel (Ch) 1 of an STS channel, and transmitted. The VT channel A is then dropped at node ID3. The VT channel B passes through node ID3, and is dropped at node ID2.
Further, a VT channel C is added at node ID3, and mapped into a channel (Ch) 1-2 of the STS channel, and thereafter dropped at node ID1 via node ID2.
In such a configuration of normal condition, when communication on both before and after node ID3 becomes disabled, the VT channels A, B are bridged from the STS channels (Ch) 1 to channels (Ch) 25 at node ID4, as shown in FIG. 4B ({circle around (1)}).
Also, in node ID2, channels (Ch) 25 are switched to channels (Ch) 1 ({circle around (2)}). With this, the VT channel A is dropped at node ID1, which produces a misconnection. Because the misconnection on the VT channel A is detected, the STS channel (Ch 1) is squelched.
At this time, in node ID1, an AIS (alarm indication signal) is inserted against the misconnection on the VT channel A. However, although line relief must be performed intrinsically for the VT channel B, the relief processing is not performed on the channel B. Instead, the AIS insertion processing is performed in node ID2 ({circle around (4)}), caused by the squelch processing ({circle around (3)}) for the STS channel 1 (Ch 1).
Namely, the VT channel B, on which the failure is to be evaded, is squelched. As a result, unnecessary service interruption is produced against the network user.
Because of this, it is required to provide identical settings of a source node and a destination node against the entire VT-level channels having been bundled into an identical STS channel. A relay node relaying a channel is not permitted to add or drop to/from a VT level channel.
To avoid this, there has been proposed a method of newly configuring a VT-level squelch table, in which channel processing for the VT level independent of the processing for the STS level is performed. (Refer to patent document Japanese Unexamined Patent Application No. 2001-186159)
However, supposing the higher-order transmission rate remains unchanged, it is necessary to prepare squelch table data (two bytes are required for each channel) of maximum 28 times as large as the size of the existing squelch table, for implementation, accommodation and processing, because 28 VT1.5 channels (VTs of 1.5 Mbps each) are included in one STS channel.
Further, 5, 376 VT1.5 channels must be processed for the equipment supporting a 10-Gbps BLSR, or 21,504 VT1.5 channels must be processed for supporting a 40-Gbps BLSR.
Therefore, when implementing the aforementioned proposal, it is required to expand hardware memory capacity and software processing capacity, as well as a vast amount of man-hours for software development and evaluation. In addition, when performing the squelch processing by software, increased loads produced by the concentrated processing in one node may undesirably cause a problem on the overall network performance.